In a nuclear magnetic resonance spectrometer (hereinafter referred simply to as "NMR"), stabilization of the magnetic field is essential. Hereinafter the explanation on this necessity will be explained in the case of NMR for measurement of a hydrogen nucleus (magnetic field=14 KG, frequency=60 MHz). The range of distribution of the NMR signals of the hydrogen nucleus is about 10 ppm. This corresponds to 600 Hz in terms of frequency. On the other hand, the half value width of the resonance absorption spectrum is about 0.5 Hz. In order to obtain a stable spectrum, therefore, stability of about 0.1 Hz is necessary between the resonance frequency and the magnetic field. This value is about 10.sup.-9 of 60 MHz. The frequency source of NMR in this instance is a crystal oscillator having a stability of about 10.sup.-6 /.degree.C. and stability of magnetic force of the magnet is about 10.sup.-4 /.degree.C. Hence, the frequency source and the magnet are placed inside a thermostatic oven that is subjected to high accuracy temperature control. Further, magnetic shielding is applied to them. As a result, it becomes possible to obtain such a stability as required for the frequency source against the external temperature change or against the disturbance to the magnetic field. Yet, it is not possible to obtain sufficient stability required for the magnet. Hence, the variation of the NMR signal of a specific reference material is fed back as an error signal to the magnet or to the frequency source. The method of stabilizing the magnetic field using this feed-back is called the lock system of NMR. This lock system is divided into two groups; one being a homonuclear lock system using a specific NMR signal of a nuclide to be measured and the other being a heteronuclear lock system using an NMR signal of nuclide that is different from the nuclide to be measured.
In the abovementioned NMR for measuring the hydrogen nucleus, problems with the conventional method will now be explained with reference to the heteronuclear system using a solvent for duterium, by way of example. The resonance frequency of the deuterium nucleus is 9.2 MHz at the magnetic field intensity of 14 KG. Hence, a radio frequency of 9.2 MHz is irradiated onto a sample, which is placed in the magnetic field of 14 KG. The solvent for the sample has the deuterium nucleus. On the other hand, a modulation magnetic field of f KHz is applied to the sample. This frequency f is so set that (9.2 MHz+f KHz) becomes a resonance frequency of the deuterium nucleus used for locking. The NMR signal of the deuterium nucleus is converted into an audio frequency by a phase detector of 9.2 MHz. Further, the signal of this audio frequency is detected by a phase detector of f KHz. The output of this detector is an error signal corresponding to a deviation of the phase of f KHz. The magnetic field inensity is controlled by feeding back this error signal to a feed-back coil of the magnet.
Now, the distribution range of the NMR signal of the deuterium nucleus in this instance is about 10 ppm (about 90 Hz). Accordingly, the resonance frequency of the deuterium nucleus varies depending on the kind of the solvent for deuterium used for locking. For this reason, an oscillator used for modulating the magnetic field is a voltage control oscillator. The oscillation frequency of this voltage control oscillator is changed by changing the value of a variable resistor. When the kind of the solvent for deuterium nucleus is changed, therefore, this variable resistor is adjusted.
The stability of the voltage control oscillator in this case is extremely low in comparison with that of the crystal oscillator. Hence, the accuracy in stabilizing the magnetic field becomes lower. In recent year, a method of measurement has been employed which takes an average of the NMR signals with respect to time in order to improve the signal sensitivity. For this reason, too, requirement for the stability over an extended period is ever-increasing.
In adjusting the variable resistor, calculation of the oscillation frequency in advance or calibration operation by monitoring the position of the duterium nucleus signal is necessary.
The abovementioned problems occur not only in the heteronuclear lock system but also in the homonuclear lock system.